<!DOCTYPE html>
<script src='../../resources/testharness.js'></script>
<script src='../../resources/testharnessreport.js'></script>
<script src='../../resources/gesture-util.js'></script>
<style>
  body, html {
    margin: 0;
  }

  #scroller {
    position: absolute;
    clip: rect(0, 1000px, 1000px, 0);
    width: 300px;
    height: 300px;
    overflow: auto;
  }

  #spacer {
    height: 1000px;
  }

  #topbox {
    height: 5px;
    width: 100%;
    background-color: green;
  }
</style>
<script>
  window.onload = async () => {
    await waitForCompositorCommit();

    const scroller = document.getElementById('scroller');

    promise_test(async () => {
      // Scroll the scroller an arbitrary amount. Because of the position:
      // absolute clip, the scroller isn't yet composited.
      {
        const delta = 300;
        const location = elementCenter(scroller);
        await smoothScroll(delta,
                           location.x,
                           location.y,
                           GestureSourceType.TOUCH_INPUT,
                           'down',
                           SPEED_INSTANT);
      }

      // Removing position:absolute and clip should cause the scroller to
      // become composited.
      scroller.style.position = 'static';
      scroller.style.clip = 'unset';
      scroller.style.willChange = 'transform';

      // Scroll down a little more, this time we should be scrolling on the
      // compositor. Ensure the scroll offset was correctly propagated when the
      // compositing layer was created.
      {
        const delta = 200;
        const location = elementCenter(scroller);
        await smoothScroll(delta,
                           location.x,
                           location.y,
                           GestureSourceType.TOUCH_INPUT,
                           'down',
                           SPEED_INSTANT);
      }

      assert_equals(scroller.scrollTop, 500,
                    "Total scroll must be sum of both scrolls");
    }, 'Scroll uncomposited, composite, scroll composited maintains correct ' +
       'scroll offset');

  };
</script>
<div id="scroller">
  <div id="topbox"></div>
  <div id="spacer"></div>
</div>
